1. Field of Endeavor
The present invention relates to synthetic guide stars and more particularly to laser guide star generation.
2. State of Technology
Earth-bound astronomers have long sought to diminish the effects of the atmosphere on their observations. Stars that appear as sharp pinpricks to the eye become smeared “blobs” by the time they are imaged by large ground-based telescopes.
At the University of California's Lick Observatory on Mount Hamilton near San Jose, Calif., Laboratory researchers and their UC colleagues are installing a system on the 3-m Shane telescope that will correct these troublesome distortions. The system includes a dye laser that will create a “guide star” in the upper atmosphere and very sensitive adaptive optics that will measure and correct for atmospheric distortions. According to Scot Olivier, project scientist for the adaptive optics subsystem, the Shane is the first major astronomical telescope with such a laser system. Other groups have been using adaptive optics systems with natural guide stars. However, it turns out that not just any star will do. It must be bright enough; that is, generate enough light to serve as a reference. When observing at visible wavelengths, astronomers using adaptive optics require a fifth-magnitude star, one that is just bright enough to be seen unaided. For near-infrared observations, only a tenth-magnitude star is needed, which is 100 times fainter.
The problem, Olivier noted, is that even though there may be hundreds of thousands or even a million stars bright enough to be guide stars, they only cover a small fraction of the sky. “Many times, there just isn't a natural guide star in the area you want to observe,” he said. “This is the kind of situation where a telescope equipped with a laser guide star comes out ahead.”
Definition: Laser Guide Star—A man-made, star-like laser light source that permits an optical system (telescope) to be adjusted to cancel out the adverse effects of viewing through turbulent atmosphere. By detecting backscattered light from a laser beam fired upwards, computers and adaptive optics can compensate for the distorting effects of atmospheric turbulence on astronomical images.
Some rudimentary wavefront correction systems, which don't require lasers, are based on a mirror, which can be tilted in real-time in response to the wandering of the star image about a centroid. These minute deflections originate from the atmosphere acting like a giant prism, which varies over time bending the wavefront as a whole. It is much more difficult for such passive systems to adequately correct for higher order aberrations which change the shape of the point spread function due to multiple inhomogeneities in the atmospheric index of refraction along the light path. Laser guide star systems can offer an elegant solution to this problem by actively rather than passively sensing these inhomogeneities.
There are many prototype laser guide star systems currently in operation or in the testing phase such as the Lick Observatory system. Most are based on correcting the incoming optical wavefront using a laser to probe the index of refraction variations of the atmosphere along the path. With this knowledge, computers and high speed deformable or tiltable mirrors can be used to reverse these wavefront distortions.
Laser guide star efforts have generally focused on two methods of creating artificial stars. The first method uses visible or ultraviolet light to reflect off air molecules in the lower atmosphere from fluctuations (Rayleigh scattering), creating a star at an altitude of about 10 km. The other method uses yellow laser light to excite sodium atoms at about 90 km. The sodium-layer laser guide star turns out to be crucial for astronomy, because astronomers need large telescopes to see objects that are very far away and therefore very dim. These large telescopes require the laser guide star to be as high as possible so that the light from the laser star and the observed object pass through the same part of the atmosphere. With a guide star at the lower elevation, the system senses and corrects for only about half of the atmosphere affecting the light from a distant object.
U.S. Pat. No. 5,412,200 for a method and apparatus for wide field distortion-compensated imaging by Geoffrey B. Rhoads, patented May 2, 1995, provide the following information beginning at column 2, line 59: “Just as adaptive optics systems have recently employed “artificial beacons” to assist in the imaging of very dim objects, so too can this invention utilize various forms of this concept as described herein. Artificial beacons can be employed when the brightness of an object under study is insufficient or inappropriate to provide photons to a wavefront sensor. The beacons are generally laser beams directed along a close line of sight to the object, generating backscatter photons which will undergo largely similar phase distortions as the photons from the object under study, and thus they can be used to deduce the phase distortions applicable to the object photons.”
U.S. Pat. No. 5,448,053 for a method and apparatus for wide field distortion-compensated imaging by Geoffrey B. Rhoads, patented Sep. 5, 1995, provides the following abstract: “An imaging system for measuring the field variance of distorted light waves collects a set of short exposure “distorted” images of an object, and applies a field variant data processing methodology in the digital domain, resulting in an image estimate which approaches the diffraction limited resolution of the underlying physical imaging system as if the distorting mechanism were not present. By explicitly quantifying and compensating for the field variance of the distorting media, arbitrarily wide fields can be imaged, well beyond the prior art limits imposed by isoplanatism. The preferred embodiment comprehensively eliminates the blurring effects of the atmosphere for ground based telescopes, removing a serious limitation that has plagued the use of telescopes since the time of Newton.”
U.S. Pat. No. 6,084,227 for a method and apparatus for wide field distortion-compensated imaging by Geoffrey B. Rhoads, patented Jul. 4, 2000, provide the following information beginning at column 1, line 15: “The limitations on imaging system performance imposed by a turbulent media, most simply described as ‘blurring,’ are well known, particularly in applications using medium to large aperture telescopes in the open atmosphere. These limitations have not only led to a variety of system solutions that will be discussed as prior art, but have played a major role in the decision to launch space based telescopes and have led to serious postulations of lunar based observatories. For a large aperture telescope—generally greater than a 10 centimeter diameter for the visible light region—which is otherwise constructed to a precision commonly referred to as “near diffraction limited,” the overall ability to resolve objects obscured by a turbulent atmosphere is limited by the turbulence rather than by the instrument. For the visual band of light once more, it is quite typical for a 1 meter aperture telescope to have ten times worse resolving power due to the turbulence, while a 10 meter aperture telescope can be 100 times or more worse than its innate “diffraction limit.” The exact numbers for any given telescope on any given night are a function of many variables, but this general level of degradation is widely recognized. As importantly, this atmospheric blurring directly leads to a loss in effective sensitivity of these large aperture imaging systems, which either renders dim objects just too dim to be seen or forces greatly extended exposure times, ultimately limiting the number of objects that can be imaged during a given length of usage time. The prior art for addressing this problem and trying to alleviate it can be generally categorized into the following well known areas: 1) Telescope Placement; 2) Adaptive Optics Systems; and 3) Speckle Inferometric Systems.”